Conventionally, disposable body warmers enclosing an exothermic composition, which generates heat by contact with air and is enclosed in an air-permeable flat bag (inner bag), have been known as a tool for warming the body. Such disposable body warmers are frequently used because they are excellent in portability, safety, convenience, and the like, and are inexpensive. In an unused state, the inner bag of the disposable body warmer is packaged in an airtight flat bag (outer bag) so as to prevent the exothermic composition from contacting with air. For example, as shown in FIG. 17, a disposable body warmer 100 formed of an outer bag 101 having a single accommodation unit 102, which contains a single inner bag 103, has been generally known.